Aspidogastrea
Encyclopedia
The Aspidogastrea is a small group of flukes comprising about 80 species
. It is a subclass of the trematoda, and sister group to the Digenea
. Species range in length from approximately one millimeter to several centimeters. They are parasites of freshwater
and marine
molluscs and vertebrate
s (cartilaginous
and bony
fish
es and turtle
s). Maturation may occur in the mollusc or vertebrate
host. None of the species has any economic importance, but the group is of very great interest to biologists because it has several characters which appear to be archaic.
with short protrusions, so-called "microtubercles".
e of some species have ciliated patches. Those of Multicotyle purvisi have four patches on the anterior side of the posterior sucker and six at the posterior side, those of Cotylogaster occidentalis have an anterior ring of eight and a posterior ring of six, while larvae of Aspidogaster conchicola, Lobatostoma manteri, Rugogaster hydrolagi lack cilia altogether. Larvae of some species hatch from eggs, others do not.
s as an excretory mechanism
. The two excretory bladder
s are located dorsal
ly, on the anterior side of the posterior sucker, connected to ducts, and three flame cell "bulbs" on each side of the body; the ducts contain cilia to aid the flow of excreta.
of extraordinary complexity, greater than that of related free-living forms, and a great number of sensory receptor
s of many different types. The nervous system is of great complexity, consisting of a great number of longitudinal nerve
s (connectives) connected by circular commissures. The brain
(cerebral commissure) is located dorsally, in the anterior part of the body, the eyes dorsally attached to it. A nerve from the main connective enters the pharynx and also supplies the intestine. Posteriorly, the main connective enters the sucker.
Sensory receptors are scattered over the ventral and dorsal surface, the largest numbers occurring on the ventral surface, at the anterior end and on the posterior sucker. Electron-microscopic
studies revealed 13 types of receptors.
is much simpler than that of digenea
n trematodes, including a mollusc and a facultative or compulsory vertebrate
host
. There are no multiplicative larval stages in the mollusc host, as known from all digeneans.
Host specificity of most aspidogastreans is very low, i.e., a single species of aspidogastrean can infect a wide range of host species, whereas a typical digenean trematode is restricted to few species (at least of molluscs). For example, Aspidogaster conchicola infects many species of freshwater
bivalves belonging to several families, as well as snails, many species of freshwater fish
es of several families, and freshwater tortoise
s.
Life cycles have been elucidated for a number of species. Lobatostoma manteri is an example of a species which has obligate vertebrate hosts. Adult worm
s live in the small intestine
of the snubnosed dart, Trachinotus blochi (Teleostei
, Carangidae
), on the Great Barrier Reef
. They produce large numbers of eggs
which are shed in the faeces. If eaten by various prosobranch snails, larvae hatch in the stomach
, and—depending on the species of snail—stay there or migrate to the digestive
gland where they grow up to the preadult stage which has all the characteristics of the adult including a testis and ovary.
d in various, complex, media
. However, their parasitic stages die soon in water. Aspidogastreans may survive for many days or even weeks outside a host in simple physiological saline
solution). For example, adult A. conchicola survived in water for a fortnight, and in a mixture of water and saline solution for up to five weeks. L. manteri extracted from fish
could be kept alive for up to 13 days in dilute sea water in which they laid eggs containing larvae infective to snails. This has led to the suggestions that aspidogastreans are archaic trematodes, not yet well adapted to specific hosts, which have given rise to the more "advanced" digenean trematodes, and that the complex life cycles of digenean trematodes have evolved from the simple ones of aspidogastreans.
Synapomorphies
of the trematodes are presence of a Laurer's Canal
, a posterior sucker (transformed to an adhesive disc in the Aspidogastrea), and life cycles involving molluscs and vertebrates. DNA studies have consistently supported this sister group relationship. The question of whether vertebrates or molluscs are the original hosts of the trematodes, has not been resolved.
This view is supported by the evolutionary relationships of the hosts which these two subclasses utilise. The hosts of aspidogastreans include chondrichthyan fishes
(shark
s, rays
and chimaera
s), a group that is 450 million years old, whereas the digeneans, are known from teleost fishes (210 million years old) as well as from various "higher" vertebrates; very few species have invaded chondrichthyans secondarily.
Gibson further recognized two orders, the Aspidogastrida with the single family Aspidogastridae, and the Stichocotylida including the Stichocotylidae, Multicalycidae and Rugogastridae. However, similarities between species of these two orders are so great that distinction at the level of orders does not seem justified.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
. It is a subclass of the trematoda, and sister group to the Digenea
Digenea
Digenea is a subclass within the Platyhelminthes consisting of parasitic flatworms with a syncytial tegument and, usually, two suckers, one ventral and one oral. Adults are particularly common in the digestive tract, but occur throughout the organ systems of all classes of vertebrates...
. Species range in length from approximately one millimeter to several centimeters. They are parasites of freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
and marine
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
molluscs and vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s (cartilaginous
Chondrichthyes
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
and bony
Actinopterygii
The Actinopterygii or ray-finned fishes constitute a class or sub-class of the bony fishes.The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines , as opposed to the fleshy, lobed fins that characterize...
fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
es and turtle
Turtle
Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...
s). Maturation may occur in the mollusc or vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
host. None of the species has any economic importance, but the group is of very great interest to biologists because it has several characters which appear to be archaic.
Shared characteristics
Shared characteristics of the group are a large ventral disc with a large number of small alveoli ("suckerlets") or a row of suckers and a tegumentTegument (Helminth)
Tegument is a terminology in helminthology for the name of the outer body covering among members of the phylum Platyhelminthes. The name is derived from a Latin word tegumentum or tegere, meaning "to cover". It is characteristic of all flatworms including the broad groups of tapeworms and flukes...
with short protrusions, so-called "microtubercles".
Larval physiology
LarvaLarva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e of some species have ciliated patches. Those of Multicotyle purvisi have four patches on the anterior side of the posterior sucker and six at the posterior side, those of Cotylogaster occidentalis have an anterior ring of eight and a posterior ring of six, while larvae of Aspidogaster conchicola, Lobatostoma manteri, Rugogaster hydrolagi lack cilia altogether. Larvae of some species hatch from eggs, others do not.
Excretory system
Like most platyhelminthes, aspidogastreans use flame cellFlame cell
right|frameA flame cell is a specialized excretory cell found in the simplest freshwater invertebrates, including flatworms , rotifers and nemerteans; these are the simplest animals to have a dedicated excretory system. Flame cells function like a kidney, removing waste materials...
s as an excretory mechanism
Excretion
Excretion is the process by which waste products of metabolism and other non-useful materials are eliminated from an organism. This is primarily carried out by the lungs, kidneys and skin. This is in contrast with secretion, where the substance may have specific tasks after leaving the cell...
. The two excretory bladder
Gas bladder
The swim bladder, gas bladder, fish maw or air bladder is an internal gas-filled organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth without having to waste energy in swimming...
s are located dorsal
Dorsum (biology)
In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...
ly, on the anterior side of the posterior sucker, connected to ducts, and three flame cell "bulbs" on each side of the body; the ducts contain cilia to aid the flow of excreta.
Nervous system
Aspidogastreans have a nervous systemNervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
of extraordinary complexity, greater than that of related free-living forms, and a great number of sensory receptor
Sensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...
s of many different types. The nervous system is of great complexity, consisting of a great number of longitudinal nerve
Nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
s (connectives) connected by circular commissures. The brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
(cerebral commissure) is located dorsally, in the anterior part of the body, the eyes dorsally attached to it. A nerve from the main connective enters the pharynx and also supplies the intestine. Posteriorly, the main connective enters the sucker.
Sensory receptors are scattered over the ventral and dorsal surface, the largest numbers occurring on the ventral surface, at the anterior end and on the posterior sucker. Electron-microscopic
Electron microscope
An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...
studies revealed 13 types of receptors.
Life cycles
Their life cycleBiological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
is much simpler than that of digenea
Digenea
Digenea is a subclass within the Platyhelminthes consisting of parasitic flatworms with a syncytial tegument and, usually, two suckers, one ventral and one oral. Adults are particularly common in the digestive tract, but occur throughout the organ systems of all classes of vertebrates...
n trematodes, including a mollusc and a facultative or compulsory vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
host
Host (biology)
In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...
. There are no multiplicative larval stages in the mollusc host, as known from all digeneans.
Host specificity of most aspidogastreans is very low, i.e., a single species of aspidogastrean can infect a wide range of host species, whereas a typical digenean trematode is restricted to few species (at least of molluscs). For example, Aspidogaster conchicola infects many species of freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
bivalves belonging to several families, as well as snails, many species of freshwater fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
es of several families, and freshwater tortoise
Tortoise
Tortoises are a family of land-dwelling reptiles of the order of turtles . Like their marine cousins, the sea turtles, tortoises are shielded from predators by a shell. The top part of the shell is the carapace, the underside is the plastron, and the two are connected by the bridge. The tortoise...
s.
Life cycles have been elucidated for a number of species. Lobatostoma manteri is an example of a species which has obligate vertebrate hosts. Adult worm
Worm
The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...
s live in the small intestine
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...
of the snubnosed dart, Trachinotus blochi (Teleostei
Teleostei
Teleostei is one of three infraclasses in class Actinopterygii, the ray-finned fishes. This diverse group, which arose in the Triassic period, includes 20,000 extant species in about 40 orders; most living fishes are members of this group...
, Carangidae
Carangidae
Carangidae is a family of fish which includes the jacks, pompanos, jack mackerels, and scads.They are marine fish found in the Atlantic, Indian and Pacific Oceans...
), on the Great Barrier Reef
Great Barrier Reef
The Great Barrier Reef is the world'slargest reef system composed of over 2,900 individual reefs and 900 islands stretching for over 2,600 kilometres over an area of approximately...
. They produce large numbers of eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
which are shed in the faeces. If eaten by various prosobranch snails, larvae hatch in the stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
, and—depending on the species of snail—stay there or migrate to the digestive
Digestion
Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones....
gland where they grow up to the preadult stage which has all the characteristics of the adult including a testis and ovary.
Evolutionary relationships
Digenean trematodes have been cultureCulture
Culture is a term that has many different inter-related meanings. For example, in 1952, Alfred Kroeber and Clyde Kluckhohn compiled a list of 164 definitions of "culture" in Culture: A Critical Review of Concepts and Definitions...
d in various, complex, media
Growth medium
A growth medium or culture medium is a liquid or gel designed to support the growth of microorganisms or cells, or small plants like the moss Physcomitrella patens.There are different types of media for growing different types of cells....
. However, their parasitic stages die soon in water. Aspidogastreans may survive for many days or even weeks outside a host in simple physiological saline
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
solution). For example, adult A. conchicola survived in water for a fortnight, and in a mixture of water and saline solution for up to five weeks. L. manteri extracted from fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
could be kept alive for up to 13 days in dilute sea water in which they laid eggs containing larvae infective to snails. This has led to the suggestions that aspidogastreans are archaic trematodes, not yet well adapted to specific hosts, which have given rise to the more "advanced" digenean trematodes, and that the complex life cycles of digenean trematodes have evolved from the simple ones of aspidogastreans.
Synapomorphies
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
of the trematodes are presence of a Laurer's Canal
Laurer's canal
Laurer's canal is a part of the reproductive system of trematodes, analogous to the vagina. In the Digeneans it opens from the dorsal surface of the worm, and in the Aspidogastreans ends in a blind ended sac...
, a posterior sucker (transformed to an adhesive disc in the Aspidogastrea), and life cycles involving molluscs and vertebrates. DNA studies have consistently supported this sister group relationship. The question of whether vertebrates or molluscs are the original hosts of the trematodes, has not been resolved.
This view is supported by the evolutionary relationships of the hosts which these two subclasses utilise. The hosts of aspidogastreans include chondrichthyan fishes
Chondrichthyes
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
(shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
s, rays
Batoidea
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...
and chimaera
Chimaera
Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, ratfish , spookfish , or rabbitfishes...
s), a group that is 450 million years old, whereas the digeneans, are known from teleost fishes (210 million years old) as well as from various "higher" vertebrates; very few species have invaded chondrichthyans secondarily.
Families within the Aspidogastrea
Rohde (2001) distinguish four families of Aspidogastrea:- The RugogastridaeRugogastridaeThe Rugogastridae are a taxonomic family of flatworms that consist of a single genus, Rugogaster, with only two species. The rugogastridae are parasites, and infect the rectal glands of various species of holocephalan fish...
include a single genus, Rugogaster, with two species from the rectal glands of holocephalan fishes. It is characterised by a single row of rugae (transverse thickenings of the body surface), numerous testes, and two caecaCecumThe cecum or caecum is a pouch, connecting the ileum with the ascending colon of the large intestine. It is separated from the ileum by the ileocecal valve or Bauhin's valve, and is considered to be the beginning of the large intestine. It is also separated from the colon by the cecocolic...
. Species of all other families have a single caecum and either one or two testes. - The StichocotylidaeStichocotylidaeStichocotylidae is a monospecific family of flatworms comprising the single species Stichocotyle nephropis, which is an internal parasite of elasmobranch fish, originally described from the Norway lobster, Nephrops norvegicus, by J. T. Cunningham in 1884. This flatworm is distinguished by a single...
include the single species Stichocotyle nephropis from the intestine of elasmobranchs. It has a single ventral row of well separated suckers. - The Multicalycidae include the single genus Multicalyx from the intestine of holocephalanHolocephaliThe subclass Holocephali is a taxon of cartilaginous fish, of which the order Chimaeriformes is the only surviving group.Holocephali has an extensive fossil record that starts during the Devonian period. However, most fossils are teeth, and the body forms of numerous species are not known, or, at...
s and elasmobranchs. It is characterised by a single ventral row of alveoli. - The Aspidogastridae includes species infecting molluscs, teleosts and turtles. The ventral adhesive disc bears either three or four rows of alveoli. Rohde distinguishes three subfamilies of Aspidogastridae, the Rohdellinae, Cotylaspidinae and Aspidogastrinae.
Gibson further recognized two orders, the Aspidogastrida with the single family Aspidogastridae, and the Stichocotylida including the Stichocotylidae, Multicalycidae and Rugogastridae. However, similarities between species of these two orders are so great that distinction at the level of orders does not seem justified.
External links
- Aspidogastrea knol Ihttp://knol.google.com/k/klaus-rohde/the-aspidogastrea-a-parasitological/xk923bc3gp4/13#
- Aspidogastrea konl IIhttp://knol.google.com/k/klaus-rohde/the-aspidogastrea-a-parasitological/xk923bc3gp4/15
- Aspidogastrea knol IIIhttp://knol.google.com/k/klaus-rohde/the-aspidogastrea-a-parasitological/xk923bc3gp4/16